1. Field of the Invention
The present invention relates generally to the field of downlink timeslot power control in TDMA (Time Division Multiple Access) based wireless communication networks having mobile stations.
2. Description of Related Art
In a telecommunications system, e.g., a cellular radio system, any one of several access strategies may be employed, for example, FDMA (Frequency Division Multiple Access), CDMA (Code Division Multiple Access), or TDMA.
In North America, a digital cellular radiotelephone system using TDMA is called D-AMPS (Digital Advanced Mobile Phone System), some of the characteristics of which are specified in the TIA/EIA-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system, using direct sequence CDMA, is specified by the TIA/EIA/IS-95 standard. There are also frequency hopping TDMA and CDMA communication systems, one of which is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for PCS (Personal Communication Services) systems.
In an FDMA based system, the frequency spectrum is divided into a number of disjunctive frequency bands, where each band serves as a separate radio channel. In a system that employs CDMA, spreading codes are used to distinguish the various radio channels.
In a TDMA based system, however, the time domain is divided into time frames. Each time frame is then further divided into a number of timeslots, for example, three timeslots. Thus, each carrier frequency-timeslot combination constitutes a different physical channel over which a communications signal burst can be transmitted. In a cellular radio telecommunications system, a communications signal burst transmitted from a mobile station to a corresponding radio base station is referred to as an uplink burst. In contrast, a communications signal burst transmitted from the radio base station to the mobile station is referred to as a downlink burst.
FIG. 1 illustrates a conventional TDMA cellular radio system including cells C1-C10 and base stations B1-B10, one base station per cell. The base stations are typically situated in the vicinity of the cell center and have omnidirectional antennas. The base stations of adjacent cells may, however, be co-located in the vicinity of cell borders and have directional antennas, as is well known to those skilled in the art. Each base station typically supports multiple carrier frequencies, and adjacent base stations have different sets of carrier frequencies to prevent or reduce interference.
The system also includes mobile stations M1-M10 that are movable within a cell and from one cell to another. An MSC (Mobile Switching Center) is connected to the base stations by, for example, cables or fixed radio links. The MSC is also connected to a fixed public switching telephone network or a similar fixed network with ISDN facilities. In addition to the MSC illustrated in FIG. 1, there may also be other mobile switching centers.
Power control, or in other words the ability to modify or adjust the power levels associated with communications signal bursts, particularly, downlink communications signal bursts transmitted from a base station to a mobile station, is important in a telecommunications system to ensure that the signal quality associated with a given channel is adequate. Power control also helps improve the spectral efficiency of the system as a whole by a) balancing average, system-wide signal quality and system capacity, and b) effectively limiting the emitted energy that acts as interference at radio connections with other mobile stations, or in other words, reducing interference from co-channels.
Downlink power control relies on received signal quality and received signal strength as reported from the mobile station in order to regulate the base station output power so that minimum requirements for speech quality are fulfilled but energy emitted is minimized to keep interference low. For this purpose, algorithms are implemented in the base stations which use measurement results transmitted from the mobile station. The parameter can include, for example, the measured quality and the measured RSSI (Received Signal Strength Information) of the downlink data, and radio network management parameters transmitted from the MSC, such as acceptable speech quality. A more detailed description of downlink power control is provided in U.S. patent application Ser. No. 09/399,764, filed Sep. 21, 1999, now U.S. Pat. No. 6,529,494 which is incorporated herein by reference.
In accordance with the TDMA standard, IS-136 Rev. A, with which a large number of mobile stations comply, downlink transmission power level remains constant throughout each time frame. Thus, a mobile station receiving a downlink burst during a given timeslot expects the power level of the received burst to remain constant, or nearly constant, over the timeslot, notwithstanding attenuations due to fading. However, it is highly probable that the TDMA standard (or other future standards) will soon incorporate downlink power control, where the transmission power level from timeslot to timeslot may be adjusted, to achieve better signal quality and spectral efficiency. This adjustment technique is often referred to as Time Slot Power Control (TSPC).
If the TDMA standard incorporates downlink power control, as introduced in the ANSI 136 rev. A specification, mobile stations which are designed in accordance with the present TDMA standards (i.e., legacy mobile stations), particularly those mobile stations that are not designed to measure RSSI (Received Signal Strength Information) during the timeslot in which they are receiving downlink data, may be unable to accurately measure and report RSSI.
It has been shown that mobile phones made with the earlier IS-136 and IS-54B specifications can also handle TSPC (TimeSlot Power Control), i.e., downlink power control in each timeslot, given that certain limitations on signal strength variation between timeslots are observed. Note that IS-136.2, rev. A specifies (see, e.g., chapter 2.4.5.4.1.2.1) how the RSSI reported by the mobile station to the base station in the Channel Quality Message, is obtained. In accordance with IS-136.2, Rev A, RSSI values for 25 frames received during one second, are summed and then divided by 25 to obtain an average RSSI that is reported by the mobile station to the base station. However, the standard does not specify which part or parts of a received burst or frame shall be measured to obtain the RSSI value for that frame. As indicated above, mobile phones from different manufacturers often use different methods, for example by measuring at different times in a frame, to determine downlink channel signal strength. Thus, when TSPC is used to control downlink power in each timeslot so that downlink channel signal transmission power levels vary during a frame, mobile stations from different manufacturers will report different, and likely inaccurate, RSSIs for the same timeslots.
In summary, although TSPC can provide many advantages, not all mobile stations will be equipped to support TSPC. The PV (Protocol Version) of a mobile station indicates the air interface protocol standard (such as TIA/EIA 136 Rev. A) that the mobile station is capable of supporting. Since neither the current nor the previous versions of the air interface protocol standard support downlink power regulation on a per timeslot basis, mobile stations with older PVs may not be able to handle TSPC. The TSPC feature handles this by discerning whether any mobile stations using a particular carrier or communication channel are incapable of supporting TSPC. This can be done, for example, using PV information provided by the mobile stations.
If any one of several mobile stations using a carrier cannot support TSPC, then TSPC is turned off for that carrier and only BSPC (Base Station Power Control, or carrier based power control) is used. Thus, the system will have to operate on that carrier using a protocol that all of the mobile stations using the carrier can support or comply with, even though some of the mobile stations support more advanced protocols and associated features. In other words, where some of the mobile stations on the carrier are more advanced than other mobile stations on the carrier, the advanced capabilities cannot be used, because communications on the carrier must operate at the level of the least capable mobile station. Unfortunately, BSPC is less efficient in reducing co-channel interference than TSPC. This makes it more difficult to transition from a 7/21 cell reuse plan to a 4/12 cell reuse plan, and in a worst case will prevent the transition. To further complicate matters, a new air interface protocol standard will likely be coming soon, and will probably implement TSPC differently from the way it is implemented now.
Thus, future TDMA systems will have at least three types of mobile station to handle: 1) old mobile stations that only support BSPC, 2) old mobile stations that support BSPC and old TSPC, and 3) new mobiles that support both BSPC and new TSPC. Accordingly, one challenge will be, how to most efficiently interact with different types of mobile station.
In accordance with an exemplary embodiment of the invention, mobile stations are sorted according to their capabilities, and are packed by an enhanced FRP (Frequency Re-Packing) function or an enhanced IDCS (Interference Driven Channel Selection) function so mixing of mobiles having different capabilities on the same carrier will be avoided as far as possible.
In accordance with exemplary embodiments of the invention, the sorting can be done using information that indicates or corresponds to capabilities of a mobile station. For example, mobile stations can be sorted by PV. Alternatively, mobile stations can be sorted by their respective electronic serial numbers (ESNs). The ESN can contain one or more codes indicating performance capabilities of the mobile station, or a chart or table can be consulted that maps ESN numbers to performance capabilities. Any appropriate identifier or information that indicates performance capability of the mobile station, can be used as a sort key.
In accordance with an exemplary embodiment of the invention, capabilities of each mobile station in a cell can be determined, and then a current allocation of the mobile stations among carriers in the cell can be evaluated. Next, various mobile stations in the cell can be transferred to different carriers, as appropriate, to re-allocate the mobile stations in the cell to maximize, for at least one carrier in the cell, a common capability and/or a performance level of all mobile stations on that carrier. This process can be repeated as necessary or appropriate, for example upon the setup of a new call, upon the ending of an existing call, or upon the expiration of a predetermined time period.
In accordance with an exemplary embodiment of the invention, at call setup the MSC can determine a mobile station""s PV, or other indicator of the mobile station""s performance capability. This can be done by observing a capability report from the mobile station on the DCCH (Digital Control CHannel) when the mobile station locks into a DCCH. Alternatively, when a call has been (or is being) set up on a DTC (digital voice channel), the MSC can obtain a mobile station""s PV (or other appropriate performance indicator) by sending an explicit request such as a CUR (Capability Update Request) to the mobile station and receiving a reply, via an RBS (Radio Base Station).
The MSC can then look in the cell where the call is being set up to check whether any mobile stations with the same PV or performance capability are already using a transceiver (TRX) corresponding to a base station within the system, and whether there are any idle timeslots on any of those TRXes. The MSC then sets up the new call on a timeslot in the TRX whose mobile stations have the same PV or performance capability as the new mobile station.
If no timeslot is idle or available in an optimal TRX, then the call can be set up on a sub-optimal TRX, for example on a TRX having one or more mobile stations whose PV or performance capability is different from the PV or performance capability of the new mobile station. The mobile station can then be transferred later to an optimal TRX when a timeslot becomes available, as described below.
In accordance with another exemplary embodiment or aspect of the invention, a TRX (e.g., TRX-1) having ongoing calls for mobile stations with different PVs, some of which do not support TSPC, is monitored. Since some of the mobile stations cannot support TSPC, the TRX-1 is prevented from running TSPC. As soon as a call ends on another TRX (e.g., TRX-2), the MSC checks to see if the PVs or performance capabilities of any mobile stations still using TRX-2 have the same PV or performance capability as any of the mobiles on TRX-1. If yes, then an intracell handoff is performed from TRX-1 to TRX-2 of the mobile with a corresponding PV or performance capability, in order to pack mobiles with the same PV performance capability on the same TRX.
This same procedure can be used to transfer mobile stations from TRX-2 to TRX-1. Thus, even when both TRX-1 and TRX-2 initially each have a mixture of mobile stations having different PVs or performance capabilities, by the transferring process the mobile stations are divided into groups according to PV or performance capability so that TRX-1 has one group, and TRX-2 has the other group. In this way the full capabilities represented by each PV (or other appropriate indicator) can be used as much as possible.
If there are more than two different PVs or performance capabilities represented by the mobile stations, then mobile stations can be transferred so that all of the mobile stations on one of the TRXes have the same PV or performance capability (even if the PVs or performance capabilities on the other TRX are mixed), and at least one of the two TRXes can run TSPC. In summary, mobile stations can be transferred among TRXes so that advanced features such as TSPC can be run on as many TRXes as possible.